Gap between tunnel cover and bulkhead

I have recently bought a 110 csw with galv chassis and bulkhead. Not a complete rebuild.
There are a couple areas which don't line up properly.
As per the title there is a gap which in areas is about 5 to 10mil.
I can't tighten things any further as the plastic fasteners behind are pulling out.
Should there be or can I put in some form of gasket / seal between?
If so what can I use?

Hi, I've rebuilt a couple of Landies and if I remember correctly, there is a sort of gasket fitted from new. I suppose a metal-to-metal joint would not be very watertight on the floor...? I used a mastic strip used for remaking joints in caravan bodywork. Comes in a box and has a layer of paper wrapped between the rolled-up strip to stop it sticking together. Go to any good caravan accessory shop and you'll find it.. Peel off what you want and cut with scissors, usually about 3 to 4mm thick. If you mean not lining up properly side to side so the holes don't line up, best method is to loosen (not undo) as many of the panel fixings as you can and shuffle the panels about a bit until you get the best fit. Being something of a perfectionist, I replaced all the fixings with stainless ones, as I did with everything bolt-like on the body I could get my hands on. Makes a heck of a difference next strip-down. And you KNOW there WILL be one... Ed

Thanks for the caravan mastic idea.